As the use of palletized warehousing expands and the cost of land and warehouse facilities increases, the response has been to use higher and higher storage racks. This conserves floor space, but increases the risk of serious injury to personnel. Anything that can contribute to unbalancing of the load on the pallet can contribute to the load's instability, such as uneven settling of the load or vibration of the warehouse structure. These can be caused by external forces such as exterior traffic or nearby construction. As a result, the once vertically stacked load with adequate uniformity of weight distribution becomes unbalanced and starts to incline to the front or the back or to one side. Once this starts, the shift can be progressive, with increasing instability of the load. Another source of pallet load instability is that of personnel placing heavy articles on top of light ones, so packaged that they are unable to maintain their geometry when subject to such compressive loads. If this instability happens to be toward the warehouse aisle and the load becomes unstable and falls, the result could be serious to those in the aisle below.